In the field of powerline carrier transmission and reception, the technique of counting cycles of carrier in a time slot or window near to a zero-crossing of the power main in order to determine the bit value of a serial digital message transmitted on a R. F. Carrier superimposed on the main is known and described in British Patent No. 1592971. The ability of this technique to allow such a communications system to work in the presence of continuous noise of frequency below that of the carrier, or in the presence of discontinuous noise which cannot accumulate sufficient cycles within the window to register a bit `1` value, is an important factor in an effective implementation of remote control systems utilising power line carrier methods. Using the carrier counting technique allows a cheap and relatively broadband front end filter to be used.
In a practical implementation using a carrier frequency of 120Khz, the counting technique and bit decoding can be carried out in a custom integrated circuit and the front end filter can be a single tuned IF transformer centred on 120Khz and giving a 3dB bandwidth of approximately 10 Khz. A window can be selected of, say, 600 microseconds and 64 cycles transmitted in the window to designate a bit value of `1` and zero cycles to designate a bit valve of `0`.
The counting technique within the circuit is able to reject continuous noise below 80 Khz and also discontinuous noise at 120 Khz provided that, in the latter case, the accumulated number of cycles within the 600 microsecond receive window is less than 48. The first type of noise might be generated by televisions, switchmode power supplies or commutator noise from motors, whilst the second type of noise might be generated by triac switching in a lamp dimmer or as a result of an appliance switching on or off.
This second type of noise is characteristically a series of impulses which cause any filters on the main to ring at their resonant frequency, in this case 120 Khz. The signal is, however, discontinuous and appears as a repetitive burst of rings. The length of each burst depends upon the damping coefficient of the filter, but is assumed to be shorter than the period between impulses. Although the combination of carrier counting contained within a custom integrated circuit together with the simple front end filter is a cost effective and successful approach to power line carrier remote control, it makes sense only so long as the supply of custom chips is secure and reasonably priced. For commercial reasons, an alternative way of achieving the noise rejection characteristics described above would be desirable using commercially available components such as microprocessors.
Although there are a number of cost effective single chip microprocessors available today which could be programmed to decode messages in the format required and provide a receiver module function, none of these contain an input counter which can count in real time the number of cycles of carrier present in a narrow window near to the mains zero-crossing. To implement this function externally using standard digital counting I.C.'s would not be cost effective.